The most difficult frequencies to deal with when it comes to a very large room are the mid-bass, bass and sub-bass. From the midrange on up, you don't have to worry as much about "filling" the space. You can use more directional speakers (line arrays, horn loaded, ring radiator) and then your only worry is enough dynamic head room to reach 105 dB peaks without distortion at the seating distance.
All of this is to say that the best solution for your father is going to involve probably two very capable subwoofers - perhaps augmenting them with a mid-bass module and then focusing on getting some very efficient and directional "satellites" to handle the midrange and upper frequencies.
So the midrange and up can be handled by in-wall, on-wall or smaller sized speakers - provided they still have enough output and dynamic range for the seating distance. But this is much easier to find than trying to obtain full range speakers that can genuinely fill the room with mid-bass and lower as well while having a small footprint.
Divide and conquer
As others have said, we don't know what the budget is hear. But the bass and sub-bass are somewhat easy in the sense that between HSU, SVS, Elemental Designs, Epik or AV123, it's easy to get some massively powerful, extremely accurate subwoofers for relatively low prices. These definitely are LARGE subwoofers though, so they won't do much for eliminating footprint concerns. On the plus side though, many of them come with beautiful finish options and could most certainly be used AS furniture. Placed as a side table or a stand of some sort, they can be blended into the room as functional pieces of furniture, rather than just being a subwoofer.
Regardless of which one you chose, be SURE to decouple that beast with a Auralex GRAMMA or similar product. Decoupling the subwoofer from the floor beneath it makes a HUGE difference. Not only is the sound that you hear greatly improved, but with tremendous reduction of structure-borne transmission, the rest of the house gets far less of that "thud thud" annoying bass resonance. To me, decoupling the subwoofer is mandatory!
A mid-bass module could also go a long way to filling a very large space with stellar sound quality. The range from 50 to about 100 Hz is a critical region of the low frequencies and contains basically all of the musical nature of bass, rather than just the raw power and percussive and tactile nature of really low bass. The mid-bass is where all of the "warmth" and "fullness" of sound resides, so this is very often the range that is somewhat lacking in smaller primary speakers - even when mated with a subwoofer.
To that end, consider placing something like HSU's MBM-12 directly behind the seat. This too, should be decoupled from the floor with a GRAMMA. It is not small, but it can be somewhat "hidden" with a placement directly behind a seat, which is where it sounds best and is intended to be placed in any event.
With all of that low end taken care of, you can have your pick of slim or in-wall, or on-wall or otherwise physically smaller speakers. Again though, the thing to look for here will still be relatively high output capabilities that can handle high peak output volume levels with low distortion for a fairly long seating distance.
I would definitely give some consideration to RBH M-Series speakers or the thin towers or on-wall speakers from EMP as a great place to start!
